They've done this a couple times, and each time it only lasts 12-24 hours and then the price pops back up. I am not sure if it's Amazon's decision or WOTCs but they've been playing with the price here and there. It also popped up to $40 at one point.
I've never had a need for two PHBs, but when I see them at $20...
Buy one and donate it to a local library. Or a even school’s library.
I may buy a few copies at that price as something to donate to the 2018 Toys For Tots campaign later this year. Farming a new generation of players is an even greater feeling on top of donating to a charity that helps kids. Who knows, it might even help the diversity of the hobby since my impression is that most of the recipients are lower income minority children.
I've found that having at least two at the table is very helpful, since it allows the players and the DM to look things up in parallel.
However, there's no need for the DM to provide both, of course.
Meanwhile it seems the UK price has been put up to £42 (the DMG and MM are still £25).